Research On Room-Temperature Mixed Potential Type NO And NO₂ Sensor Based On K₂Fe₄O₇ And MOF Sensitive Electrode

The solid electrolyte gas sensor based on the principle of mixed potential has the advantages of high sensitivity,fast response-recovery characteristics,good selectivity,and good stability.It holds a key role in gas detection in many fields,such as mobile pollution source monitoring,environmental quality monitoring,medical disease diagnosis,food safety detection,and so on.For mixed potential solid electrolyte gas sensors,the continuous improvement of gas sensing performance is always the focus and difficulty of research.Meanwhile,miniaturization,low power consumption,and intelligence are also important development directions.This thesis aims to develop a miniaturized and low-power mixed potential solid electrolyte gas sensor that can detect NO and NO₂ gases with high sensitivity,high selectivity,and low detection limit for meeting application requirements in medical diagnosis or treatment and atmospheric environmental monitoring.The novel K₂Fe₄O₇is used as the solid electrolyte and MOF sensing material as the sensitive electrode.The room-temperature mixed potential NO sensor based on Ni/Fe-MOF and Ni-MOF/MWCNT sensitive electrode and the room-temperature mixed potential NO₂ sensor based on Ni-MOF（N₂ sintering）sensitive electrode are constructed respectively.The following are the main research topics:（1）Aiming at the detection of asthma breath marker NO in medical diagnosis and treatment,K₂Fe₄O₇ solid electrolyte and Ni/Fe-MOF sensitive electrode materials were prepared by hydrothermal synthesis and a room-temperature mixed potential NO sensor was constructed.The effects of microstructure,composition,and electrochemical reactivity of MOF with different Ni/Fe ion ratios on the sensing properties were investigated.The test results show that the microstructure and composition mainly affect the mass transport process of the gas,which makes the sensitivity curve of the sensor show different dynamics laws.Meanwhile,the sensor based on Ni-MOF sensitive electrode（SE）has the strongest electrocatalytic oxidation activity for NO and the weakest electrocatalytic reduction activity for O₂.Therefore,the sensor based on Ni-MOF sensitive electrode has the highest sensitivity to NO,with-19 m V/ppm（20ppb～200 ppb）and-21.5 m V/decade（500 ppb～2 ppm）,respectively.Concurrently,the sensor has a detection limit as low as 20 ppb,a response time of 22 s and a recovery time of 11 s to 100 ppb NO,and has good selectivity,long-term stability,and acceptable repeatability.In addition,the influence of humidity on sensor response was studied,and it is found that the synergistic effect of interface electrochemical activity and the number of reaction sites is the main reason for the change of sensor response signal with humidity,which provides theoretical support for improving the humidity stability of this type of sensor.Finally,the polarization curve and current-time curve are used to verify the mixed potential sensing mechanism of the solid electrolyte NO sensor based on the Ni-MOF sensitive electrode.（2）Sensitivity improvement strategy:In order to further improve the sensitivity of the mixed potential NO sensor based on K₂Fe₄O₇ and Ni-MOF sensitive electrode,multi-walled carbon nanotubes（MWCNT）were introduced into the Ni-MOF sensitive electrode,increasing the electrochemical active area and enhanced the electrocatalytic activity of the sensor to NO.The results of gas sensing characteristics show that compared with the sensor based on Ni-MOF sensitive electrode,the sensitivity of the sensor constructed by Ni-MOF/MWCNT（NM）sensitive electrode to NO is increased by 2.6 times and 1.6 times respectively in the range of low concentration and high concentration,and the response value to 500 ppb NO is increased from-8 m V to-34m V.At the same time,the sensor has good repeatability,reproducibility,and excellent selectivity in expiratory markers.In addition,the sensor has excellent reproducibility,which proves the feasibility of the sensitivity improvement strategy.（3）For the detection of ultra-low concentration NO₂ in the atmospheric environment:Ni-MOF derivatives were obtained by regulating the formation of oxygen vacancy and uncoordinated Ni ion defects in Ni-MOF through nitrogen sintering,which was marked as Ni-MOF（N₂ sintering）.A mixed potential NO₂ sensor based on K₂Fe₄O₇and Ni-MOF（N₂ sintering）sensitive electrode was constructed to improve the sensitivity and selectivity.The novel sensitive electrode material enhances the participation of water molecules in the electrochemical reaction at the three-phase interface and the electrocatalytic activity of the sensor to NO₂,while reducing the electrocatalytic activity of the sensor to NO.The results show that the NO₂ selectivity coefficient（response value:NO₂/NO）of Ni-MOF（N₂ sintering）sensor is 32 times that of Ni-MOF sensor,and the detection limit is as low as 5 ppb.In addition,the NO₂monitoring network for remote monitoring,different environment assessment,and information sharing is constructed by using the NO₂ sensor combined with a voltage signal acquisition circuit,cloud server,and user terminal.